1. Field of the Invention
The present invention relates to a control system and method for an internal combustion engine having an exhaust system provided with an exhaust gas purification device for purifying exhaust gases, and an engine control unit, which control the temperature of the exhaust system and the air-fuel ratio of exhaust gases.
2. Description of the Related Art
Conventionally, as a control system for an internal combustion engine, the present assignee has already proposed a control system disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H09-273438.This control system is for controlling the air-fuel ratio of exhaust gases flowing through an exhaust passage of the engine. A LAF sensor, an upstream catalyst, an oxygen concentration sensor and a downstream catalyst are arranged in the exhaust passage of the engine from upstream to downstream in the mentioned order. The oxygen concentration sensor detects the concentration of oxygen in exhaust gases between the upstream catalyst and the downstream catalyst. The oxygen concentration sensor has characteristics that when the exhaust gases have a richer air-fuel ratio than a stoichiometric air-fuel ratio, an output therefrom takes a high-level voltage value, whereas when the exhaust gases have a leaner air-fuel ratio than the stoichiometric air-fuel ratio, the output therefrom takes a low-level voltage value. Further, when the air-fuel ratio of exhaust gases is close to the stoichiometric air-fuel ratio, the output therefrom takes a predetermined appropriate value q between the high-level and low-level voltage values (see FIG. 2 in Japanese Laid-Open Patent Publication (Kokai) No. H09-273438).
In the control system, by an air-fuel ratio control process, described hereinafter, the output value is controlled such that it converges to the predetermined appropriate value q. First, a basic fuel injection amount Tim and a total correction coefficient KTOTAL for correcting the basic fuel injection amount Tim are calculated based on operating conditions of the engine. Next, a reference air-fuel ratio KBS is calculated according to the rotational speed of the engine and intake pressure. Further, an adaptive sliding mode control process separate from the present process is carried out to calculate a correction value Usl for causing the output value of the oxygen concentration sensor to converge to the predetermined appropriate value q, and by adding the correction value Usl to the reference air-fuel ratio KBS, a target air-fuel ratio KCMD is calculated.
Then, the target air-fuel ratio KCMD is corrected by taking charging efficiency into account, whereby a corrected target air-fuel ratio KCMDM is calculated, and further feedback coefficients #nKLAF and KFB are calculated. Subsequently, the basic fuel injection amount Tim is multiplied by the total correction coefficient KTOTAL, the corrected target air-fuel ratio KCMDM and the feedback coefficients #nKLAF and KFB, whereby a fuel injection amount #nTout for each cylinder is calculated, and further is subjected to a fuel attachment-dependent correction process. After that, a drive signal based on the fuel injection amount #nTout subjected to the fuel attachment-dependent correction process is output to a fuel injection device.
As described above, according to the above air-fuel ratio control system, an actual air-fuel ratio detected by the LAF sensor is controlled such that it converges to the target air-fuel ratio KCMD, whereby the output value of the oxygen concentration sensor is controlled such that it converges to the predetermined appropriate value q. This makes it possible to ensure excellent reduction of exhaust emissions.
According to the above-described conventional control system, when the output value of the oxygen concentration sensor has converged to the predetermined appropriate value q, the air-fuel ratio of exhaust gases is controlled to a value close to the stoichiometric air-fuel ratio, so that when the engine is a gasoline engine, it is possible to hold the output value of the oxygen concentration sensor at the predetermined appropriate value q in the substantially whole operating region, whereby it is possible to ensure excellent reduction of exhaust emissions. However, when the control range of the air-fuel ratio of a combustion air-fuel mixture of the engine is limited, for example, in the case of a diesel engine in which a combustion air-fuel mixture is controlled to a leaner value than the stoichiometric air-fuel ratio, during normal time, it is difficult to continuously control the air-fuel ratio of exhaust gases to a value close to the stoichiometric air-fuel ratio, which can result in increased exhaust emissions. Further, from a technical point of view, to ensure excellent reduction of exhaust emissions, it is necessary to quickly increase the temperature of a catalyst up to an activation temperature range for activating the catalyst, during the start of the engine, and it is necessary to hold the temperature of the catalyst within the activation temperature range, during operation of the engine. However, the above-described control system is not configured from the above technical point of view. Therefore, it sometimes takes a long time for the catalyst temperature to reach the activation temperature range during the start of the engine, and the catalyst temperature sometimes deviates from the activation temperature range during operation of the engine. In such cases, exhaust emissions are increased.